The invention pertains to a pane of transparent material with high transparency in the visible range and with very high reflectivity in the thermal radiation range and also to a process for its production.
Panes of this type should have high chemical resistance to moisture, especially to NaCl-water and SO.sub.2 -water solutions in certain concentrations.
The invention also pertains to the production of a pane of this type by a coating process based on cathode sputtering.
Windows with panes of this type should in particular prevent radiant heat from escaping from a room to the outside in winter. Known layer systems of this type are referred to as "low-e" ("low emissivity").
Conventional low-e systems consist of various categories of layers, which are designed to have different properties and which are expected to perform different functions in the system:
(a) a layer with high conductivity for electricity, often consisting of a metal such as Ag, Au, or Cu, but with a very low radiation emission coefficient, represents the actual low-e (low-emissivity) coating; PA1 (b) but because a metal layer is also highly reflective to light (a low degree of light transmission) in the visible range, additional transparent layers are deposited to reduce its reflectivity. Other functions of these transparent layers are to provide the desired color tone and to give the system a high level of mechanical and chemical resistance; PA1 (c) to protect the thin metal layer against aggressive atmospheres in the environment both during and after the production process and also to ensure the good adhesion of the adjacent oxide layer, a so-called blocker layer (barrier layer, primer layer) of a metal or suboxide is often applied to this metal layer (Ag, Au, Cu). PA1 R.sub..box-solid. =the surface resistance of the silver layer; PA1 d=the thickness of the layer; and PA1 .rho.=the resistivity. PA1 .rho..sub.K is the resistivity of a monocrystalline layer of infinite thickness; PA1 .rho..sub.F is the component of the resistivity caused by electron scattering along the layer surfaces; and PA1 .rho..sub.G is the component of the resistivity caused by electron scattering along the grain boundaries of the individual crystalline grains. PA1 first oxide layer, about 40 nm; PA1 second layer, about 4 nm; PA1 the Ag layer, about 6 nm; PA1 the blocker layer, about 1.5 nm; and PA1 the last oxide layer, about 38 nm.
To accomplish all these tasks, a conventional low-e coating is built up of the following components:
______________________________________ substrate .vertline. oxide .vertline. Ag .vertline. blocker .vertline. oxide ______________________________________
where the substrate is a pane of transparent inorganic or organic glass or a transparent organic film; Ag is an electrically conductive layer; the oxides form the antireflective coating; and the blocker forms a protective layer for the Ag and also serves as a bonding agent with respect to the oxide layer.
The light transmission of a conventional low-e coating on a 4-mm glass substrate is approximately 80-86%. The thermal transmission through a pane of glass such as this depends on the emissivity ( of the low-e coating and can be described here by means of the simple formula: EQU .epsilon..congruent.0.0141.times.R.sub..box-solid.,
where R.sub..box-solid. =.rho./d, and
The above formula describes the emissivity of a thin metal layer with sufficient accuracy as long as the value is smaller than 0.2. For the known low-e coatings, .epsilon. is approximately 0.1.
The lower the emissivity, the smaller the radiation losses through the coating. The emissivity can be suppressed either by lowering the resistivity or by increasing the thickness of the layer. When the thickness of the layer is increased, the amount of light which is absorbed also increases, which leads to an undesirable reduction in the amount of light transmitted. A reduction in the resistivity of the Ag layer, however, leads not only to a reduction in the emissivity but also to an increase in the amount of light transmitted.
The resistivity of a thin layer can be described as follows: EQU .rho.=.rho..sub.K +.rho..sub.F +.rho..sub.G,
where:
The resistivity .rho..sub.K of the very thick, monocrystalline Ag layer depends on the purity of the metal. Even a very small amount of foreign material can considerably increase the resistance of the layer. This means that the sputtering process should be carried out in a gas atmosphere of such a kind that none of its atoms is introduced into the silver layer.
The resistivity .rho..sub.F of a thin layer depends on the roughness of the layer surfaces. It is important for the lower oxide layer, on which the silver grows, to be very smooth. Thus this component of the electron scattering can be significantly reduced.
The resistivity .rho..sub.G depends on the size of the grains and on the type of grain boundaries between the individual grains. The smaller the grains and the wider and denser the grain boundaries, the greater the electron scattering. The size of the silver grains can be influenced by suitable preparation of the substrate surface. The oxide under the silver should promote the growth of the silver, which will lead to larger grains. In addition, the oxide elements may not diffuse into the silver layer. Foreign atoms diffuse into a layer primarily along the grain boundaries, which leads to an increase in density and thus to greater electron scattering.